1. Field of the Invention
The present invention relates to a machine tool, particularly to a machine tool making it possible to continuously work the back of a workpiece held by a chuck, which cannot be reached by a cutting tool, by changing the workpiece holding positions.
2. Description of the Prior Art
FIG. 1 shows an existing machine tool capable of working the back of a workpiece. A first chuck 12 for holding a workpiece 14 is set on a first chuck base 10 secured to a machine table not shown. A first tool spindle 18 holding a first cutting tool 16 for working the workpiece 14 is arranged at a position facing the workpiece 14. The first tool spindle 18 is secured onto a turret 20. The turret 20 is rotatably supported by a tool headstock 22. Moreover, it is possible to change a position where the first cutting tool 16 contacts the workpiece 14 by changing the spindle direction A of the headstock 22 and the lateral direction B perpendicular to the direction A. Thereby, it Is possible to work a desired position of a surface portion 14a of the workpiece 14 facing the first cutting tool 16. This type of working is known as front surface working. During front surface working, a rear surface portion 14b of the workpiece 14 facing the chuck 12 cannot be worked because a tool cannot enter or reach the portion 14b.
To work the rear surface portion 14b, a second chuck base 24 and a second chuck 26 are used. The second chuck base 24 is set at a position facing the first chuck base 10 and moreover, arranged so that it is able to move in the direction C of the first chuck base 10. When front surface working is completed, the tool headstock 22 temporarily moves backward and the second chuck base 24 moves toward the first chuck base 10. Then, the second chuck base 24 holds the front surface portion 14a of the workpiece 14 with the second chuck 26 and then moves backward. The front surface and the rear surface of a workpiece 14 usually have different shapes. For this reason, the first chuck has jaws which fit the rear surface of the workpiece, while the second chuck has jaws which fit the front surface of the workpiece.
The turret 20 is provided with not only the first tool spindle 18 but also a second tool spindle 28 and a second cutting tool 30 through an attachment 32. The second cutting tool 30 is turned by 180.degree. from the first cutting tool 16 by the attachment 32. This makes it possible to work the workpiece 14 held by the second chuck 26. The portion of the workpiece held by the second chuck 26 and facing the second cutting tool is the rear surface portion 14b and therefore, the portion 14b can be worked. Working of the portion 14b is called back working.
As described above, because the existing machine tool allows continuous surface working and back working, a positional deviation, which occurs when a workpiece is removed from a chuck and reversed and secured to the chuck again, is decreased. Particularly, it is possible to decrease a positional deviation in the rotational direction.
However, there is a problem that two chucks are used for one tool headstock and thereby the size of the machine tool increases. Moreover, there is a problem that, an exclusive tool is required for front surface working and rear surface working respectively but the number of tools is restricted and therefore only simple working can be carried out.